Researchers at Brown University and in Korea used focused ion beams to extract a cross-section of compressed gold nanofilm. When tips of regular, neighboring folds touched, nanopipes were created beneath the surface. Image: Kim Lab/Brown University |
Wrinkles
and folds are ubiquitous. They occur in furrowed brows, planetary
topology, the surface of the human brain, even the bottom of a gecko’s
foot. In many cases, they are nature’s ingenious way of packing more
surface area into a limited space. Scientists, mimicking nature, have
long sought to manipulate surfaces to create wrinkles and folds to make
smaller, more flexible electronic devices, fluid-carrying nanochannels
or even printable cell phones and computers.
But
to attain those technology-bending feats, scientists must fully
understand the profile and performance of wrinkles and folds at the
nanoscale, dimensions 1/50,000th the thickness of a human hair. In a
series of observations and experiments, engineers at Brown University
and in Korea have discovered unusual properties in wrinkles and folds at
the nanoscale. The researchers report that wrinkles created on
super-thin films have hidden long waves that lengthen even when the film
is compressed. The team also discovered that when folds are formed in
such films, closed nanochannels appear below the surface, like thousands
of super-tiny pipes.
“Wrinkles
are everywhere in science,” said Kyung-Suk Kim, professor of
engineering at Brown and corresponding author of the paper published in
the journal Proceedings of the Royal Society A.
“But they hold certain secrets. With this study, we have found
mathematically how the wrinkle spacings of a thin sheet are determined
on a largely deformed soft substrate and how the wrinkles evolve into
regular folds.”
Wrinkles
are made when a thin stiff sheet is buckled on a soft foundation or in a
soft surrounding. They are precursors of regular folds: When the sheet
is compressed enough, the wrinkles are so closely spaced that they form
folds. The folds are interesting to manufacturers, because they can fit a
large surface area of a sheet in a finite space.
Kim
and his team laid gold nanogranular film sheets ranging from 20 to 80
nm thick on a rubbery substrate commonly used in the microelectronics
industry. The researchers compressed the film, creating wrinkles and
examined their properties. As in previous studies, they saw primary
wrinkles with short periodicities, the distance between individual
wrinkles’ peaks or valleys. But Kim and his colleagues discovered a
second type of wrinkle, with a much longer periodicity than the primary
wrinkles—like a hidden long wave. As the researchers compressed the gold
nanogranular film, the primary wrinkles’ periodicity decreased, as
expected. But the periodicity between the hidden long waves, which the
group labeled secondary wrinkles, lengthened.
“We thought that was strange,” Kim said.
It
got even stranger when the group formed folds in the gold nanogranular
sheets. On the surface, everything appeared normal. The folds were
created as the peaks of neighboring wrinkles got so close that they
touched. But the research team calculated that those folds, if
elongated, did not match the length of the film before it had been
compressed. A piece of the original film surface was not accounted for,
“as if it had been buried,” Kim said.
Indeed,
it had been, as nano-size closed channels. Previous researchers, using
atomic force microscopy that scans the film’s surface, had been unable
to see the buried channels. Kim’s group turned to focused ion beams to
extract a cross-section of the film. There, below the surface, were rows
of closed channels, about 50 to a few 100 nanometers in diameter. “They
were hidden,” Kim said. “We were the first ones to cut (the film) and
see that there are channels underneath.”
The
enclosed nano channels are important because they could be used to
funnel liquids, from drugs on patches to treat diseases or infections,
to clean water and energy harvesting, like a microscopic hydraulic pump.
Contributing
authors include Jeong-Yun Sun and Kyu Hwan Oh from Seoul National
University; Myoung-Woon Moon from the Korea Institute of Science and
Technology; and Shuman Xia, a researcher at Brown and now at the Georgia
Institute of Technology. The National Science Foundation, the Korea
Institute of Science and Technology, the Ministry of Knowledge Economy
of Korea, and the Ministry of Education, Science, and Technology of
Korea supported the research.
